1. Field of the Invention (Technical Field)
The present invention relates to targets for physical vapor deposition and similar processes, specifically to target-backing plate assemblies and methods of manufacturing the same.
2. Background Art
Physical Vapor Deposition
Physical Vapor Deposition (PVD) and similar deposition processes are used for fabrication of thin film materials, such as, but not limited to, fabrication of thin films for xe2x80x9ccompact discsxe2x80x9d (CDs) or coatings in the semiconductor industry. Specifically, sputtering processes are often preferably involved in the coating of a semiconductor wafer or other substrate mounted within a processing chamber (a.k.a. (deposition chamberxe2x80x9d). Silicon is a preferred source material for use as a thin film in elemental and compound compositions. Much of the current application for silicon in thin films is for information display and digital storage media.
PVD and similar processes form a layer of material (coating) on a substrate surface, typically between approximately 2 microns and approximately 5 microns thick. The generic PVD process involves first removing atoms from a source, often referred to as a xe2x80x9ctargetxe2x80x9d (a first condensed phase comprising a solid or a liquid), converting them into a gas or vapor form, and then depositing these atoms through condensation on a substrate material (second condensed phase).
For example, in a typical PVD process a substrate (a.k.a. workpiece) is placed in a vacuum chamber and a very high vacuum is drawn. The vacuum chamber space is heated to between approximately 400xc2x0 F. and approximately 900xc2x0 F., depending on the specific process. (Where plasma etching is to be used. Plasma is created from an inert gas such as argon to further clean the surface of the workpiece.) Next, the source or coating metal is forced into a gas or vapor phase which is directed at a substrate to be coated. The particles condense on the substrate forming a film.
Forcing Atoms From A Target
Three methods of forcing a source metal, alloy or other compound (such as silicon) from a target are commonly used: evaporation, sputtering, and ion plating.
Evaporation comprises use of a high-current electron beam or resistive heaters to evaporate source material from, for example, a crucible. The evaporated material forms a cloud that fills the deposition chamber and then condenses onto the substrate to produce the desired film. In such a process, atoms take on a relatively low energy state (0.2 to 0.6 eV) and the deposited films, as a result, are not excessively adherent or dense. In some instances, deposition of a substantially uniform coating may require complex rotation of the substrate since the vapor flux may be localized and directional.
In a sputtering process, the surface of the source/target material is bombarded with energetic ions, usually in an ionized inert gas environment comprising, for example, argon. The physical erosion of atoms from the coating material that results from this bombardment is known as sputtering. The substrate is positioned to intercept the flux of displaced or sputtered atoms from the target. Sputtering deposits atoms with energies in the range of 4.0 to 10.0 eV onto a substrate. Of course, it may be possible to xe2x80x9cbendxe2x80x9d the line-of-sight through application of electromagnetic and/or other energy. Sputtering is, in general, more controllable than evaporation.
The third method of forcing is ion plating, which can produce superior coating adhesion by bombarding the substrate with energy during deposition process. In ion plating processes, particles accelerate towards the substrate and arrive with energy levels up to hundreds of electron volts. These atoms sputter off some of the substrate material resulting in a cleaner, more adherent deposit. This xe2x80x9ccleaningxe2x80x9d process continues as the substrate is coated. The film growth is assured when the deposition rate is faster than the sputtering or cleaning rate. In general, high gas pressure results in greater scattering of the vapor and a more uniform deposit on the substrate.
An important variation on these processes involves the introduction of a gas such as oxygen or nitrogen into the chamber to form oxide or nitride deposits, respectively. These reactive deposition processes are used to deposit films of material such as titanium nitride, silicon dioxide, and aluminum oxide. Overall, PVD processes result in a thin, uniform coating that is much less likely to require machining after application. Additionally, the specifics of the aforementioned three variations of PVD processes are by no means exclusive. For example, some PVD processes use laser ablation or pulsed laser deposition to release a controlled amount of target material in the form of a gas. Accelerated plasma can also be used in the PVD process to deliver a heat pulse to a target to release a controlled amount of gaseous target material.
Forming A Target
Before a forcing method can be utilized, a target must be formed. There are several methods of forming the target material, including but not limited to vacuum melting and powder metallurgy. Types of target materials used dictate which method is chosen. Vacuum melting is most often used for metals with relatively low melting points. In this process, the materials are mixed, melted together, and poured into a mold in the vacuum furnace which aids in outgassing volatile constituents. After cooling, the resultant ingot is machined into its final target form. The target is then attached to a backing plate which holds the target in a PVD deposition chamber.
Materials having high melting points (e.g., tungsten, molybdenum) are best formed into targets using powder metallurgy. In this process, the materials are respectively ground into powders, mixed and poured into a mold. The powder mix is then sintered or compacted by high pressure and temperature to form a solid form ingot. The ingot is then machined to a final form and joined to a backing plate. Powder metallurgy has a disadvantage in that with a low density, potentially volatile gases trapped during formation are released by sputtering, which may additionally contaminate the resultant film.
Other materials may be formed into targets through growth methods, such as Czochralski growth method (CZ). In this method, a form is slowly extracted from a molten pool of the desired material using a seed crystal. The growth occurs through a combination of capillary action and epitaxial growth. This is generally used for materials suitable for CZ growth, such as but not limited to silicon, germanium, gallium, or alloys and/or compounds thereof.
Regardless of the method used to form the target, the material is shaped into a final target form (e.g., disk, cone, square) suited for use in a particular deposition chamber. However, some materials used in sputtering are very difficult to machine and several of these are very fragile. Shaping the target is a very important variable in providing a high quality film. Unfortunately, some of the conventional PVD processes, such as those utilizing lasers, ion beams, electron beams, or electromagnetic energy, typically result in uneven wear of the target due to non-uniform heating. Non-uniform heating generally relates to the conformation of the electromagnetic field surrounding a target and/or a target""s internal temperature profile. Therefore, there is a real need in the industry for a target configuration which will help to avoid the problem of uneven wear of a target.
Use Of Backing Plates
It is common in the industry to bond the metal or metal alloy target to a backing plate. Typically, such a plate Is made from a metal or metal alloy (e.g., copper). An indium-based bonding technology is generally used to attach the target to the backing plate, creating a bonding interface. The backing plate is required in order to support the target in the chamber in which the sputtering takes place. Often, materials used to bond the target to the backing plate, or the backing plate itself, may contribute to contamination during the deposition process.
Prior Art Production Methods
In the current state of the art, planar silicon targets are produced primarily by slicing silicon boules across their diameter then machining a round tile (requiring two cuts) a or rectangular tile (requiring four more cuts) from the round cross-section. Each of the six sides of the rectangular tile or the circumference and the facing surfaces of a disc are machined. In some cases they are also ground and polished. This is a labor-and material-intensive method of production, which generates a fair amount of skeletal scrap. A real need exists for a processing method of a silicon target which is easily manufactured and resists the uneven wear (and therefore, potentially affected film quality) as described above.
The present invention comprises a target-backing plate combination assembly, comprising a configuration wherein a target having a semi-circular face is joined with a correspondingly-shaped backing plate at a semi-circular bonding interface, as well as the process for producing the assembly.
The present invention comprises a target assembly for PVD comprising a target, wherein the target has a planar face and a geometrically-shaped face, a backing plate, wherein the plate has a depression formed in an upper surface of the plate conformed for fitting receipt of the geometrically-shaped target face, and a bonding interface joining said geometrically-shaped face and said depression. The geometrically-shaped face may comprise a geometric shape selected from the group consisting of circular, oval, ellipsoidal, triangular, square, rectangular, and polygonal. A geometrically-shaped face comprises a surface area greater than a surface area of a planar face.
The target of the assembly may comprise at least one material selected from the group comprising silicon, germanium, gallium, and alloys or compounds thereof. The backing plate may comprise at least one material selected from the group comprising of copper, molybdenum, stainless steel, aluminum, and alloys thereof.
The depression of the assembly may be formed by a method selected from the group consisting of molding, pressing, and machining. The target may be formed by slicing a pre-formed boule lengthwise.
The bonding interface of the assembly may comprise a bonding layer. A bonding layer may comprise at least one material selected from the group consisting of indium, silver-tin, silver-filled epoxy, pure tine, or other suitable low melting point or curing materials.
The target dissipates radially when subjected to sputtering processes.
The present invention also includes a method of producing a target assembly comprising the steps of providing a pre-formed boule; slicing the boule lengthwise, thereby creating a semi-circular. curved face and a planar face; providing a pre-formed backing plate having a depression in an upper surface conformed for receipt of the curved face of the sliced boule; and bonding the curved face of the sliced boule to the depression of the backing plate at a bonding interface.
The method of the invention may additionally include providing a backing plate that is pre-formed, including its depression, by molding, pressing, or machining. Additionally, the bonding method may comprise chemically bonding or heat bonding. The boule may be sliced multiple times to create multiple targets.
The boule of the method may comprise a geometrically-shaped cylinder instead of a circular cylinder, may be grown by the CZ growth method, and may comprise a material selected from the group consisting of silicon, germanium, gallium, and alloys or compounds thereof.
A primary object of the present invention is to provide a target which is less likely to exhibit uneven wear during a PVD process.
Another object of the present invention is to provide a target which is easily produced with fewer machining steps.
Yet another object of the present invention is to provide a target which dissipates more completely.
A primary advantage of the present invention is less wasted target material due to greater dissipation.
Another advantage of the present invention is less contamination in the resultant film.
Another advantage is the simplified machining process.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.